1. Field of the Invention
This invention lies in the field of lipids and the phase transitions of certain lipids from the liquid crystalline phase to the gel phase. In particular, this invention addresses the problem of leakage of internal substances through the membranes of biological cells and liposomes due to passage of these bodies through the phase transition temperature and to prolonged exposure to temperatures below the phase transition temperature.
2. Description of the Prior Art
Various types of biological substances undergo thermotropic phase transitions between a gel phase and a liquid crystalline phase upon cooling down to temperatures close to but not at or below the freezing temperature. Included among such substances are plant and animal cells, bacteria and liposomes. Cooling of these materials to this phase transition region but not to the frozen state is a practical and useful means of preservation for purposes such as storage and shipping, since freezing is destructive of the cell structure and dehydration is impractical in many circumstances.
As they enter and pass through this phase transition, plant cells and liposomes have been observed to undergo leakage of their contents. In cells, this leakage destroys viability and texture, while in liposomes there is a loss in usefulness, particularly when the substance which has leaked out is a functional substance residing in the liposome interior. Liposomes are vesicles formed of ordered phospholipid bilayers encapsulating an aqueous phase. By incorporating functional molecules such as pharmaceuticals, imaging agents, skin care agents and other useful substances as solutes in the encapsulated aqueous phase, researchers have developed liposomes as useful carriers of these substances. Liposome formulations are thus of interest for such industries as the cosmetics industry and the pharmaceuticals industry, and the leakage problem is a potential obstacle to their stability during storage, shipping and handling.